Sorry if this has already been seen or even if R* have confirmed the Casino being available for use in DLC but I couldn't see any threads about this so I thought I'd post this up. I went by the casino to see if I could find any clues to it opening up sometime and I found this straight away.
I think this is a pretty big clue myself with the "ATM available inside". Also there's 16 parking spaces directly out front with one of them being a motorbike spot. Also the car park out back is pretty big, not even any NPC vehicles at the moment but if it does open, I assume we could maybe play against them or they might just be there for added effect.
The casino is surely too detailed as it is to be nothing. Again, sorry if this has been done before, I'm just surprised I've not seen a thread on this yet.